qt: add X11 support

There are two possible configurations for Qt4 installation: standard
(with X11) and embedded (for framebuffer). Both configurations cannot be
used together for one installation and some options are specific to one
configuration or the other.

Add a choice to select the standard or the embedded installation to add
X11 support.

The integration is done in order to make maximum use of the current
configuration logic, for example by using the same mkspecs files.

Signed-off-by: Hadrien Boutteville <hadrien.boutteville@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>